使用PHP FORM更新数据库中的图像

我正在创建一个内容管理系统，以便可以轻松地更新数据库。我创建了一个更新页面，用户可以编辑数据库中的条目，他们可以上传新图像并编辑任何字段，而不会出现任何问题。我遇到的问题是，如果你只想编辑，比如说只想编辑"描述"字段中的文本，但你不想更改图像，当表单提交时，它会用一个空白条目覆盖图像字段。如果没有选择其他文件，我想做的是保留原始图像，但如果我上传了一个新图像，它将被新图像替换。按照目前的形式，更新数据库条目的唯一方法是在每次更新任何内容时重新上传图像。不过，当网站运行时，这是不可行的，因为它需要从不同的设备进行更新，而且图像不会出现在每个设备上。如果这听起来像是一个愚蠢的问题，我很抱歉，但我是php的新手，我已经被困在这个问题上好几天了。我相信这可能很简单，但这是我上传的脚本：

$editFormAction = $_SERVER['PHP_SELF'];
if (isset($_SERVER['QUERY_STRING'])) {
/* upload image script */
$thumb=basename($_FILES['thumb']['name']);
if(move_uploaded_file($_FILES['thumb']['tmp_name'],"../shopImages/".$thumb))
$image1=basename($_FILES['image_1']['name']);
if(move_uploaded_file($_FILES['image_1']['tmp_name'],"../shopImages/".$image1))
$image2=basename($_FILES['image_2']['name']);
if(move_uploaded_file($_FILES['image_2']['tmp_name'],"../shopImages/".$image2))
/*end of script*/
$editFormAction .= "?" . htmlentities($_SERVER['QUERY_STRING']);
}

if ((isset($_POST["MM_update"])) && ($_POST["MM_update"] == "form1")) {
 $updateSQL = sprintf("UPDATE tbl_products SET thumb=%s, image_1=%s, image_2=%s, title=%s, `desc`=%s, category_id=%s, status=%s, shipping_band=%s, price=%s WHERE product_id=%s",
               GetSQLValueString($thumb, "text"),
               GetSQLValueString($image1, "text"),
               GetSQLValueString($image2, "text"),
               GetSQLValueString($_POST['title'], "text"),
               GetSQLValueString($_POST['desc'], "text"),
               GetSQLValueString($_POST['category_id'], "int"),
               GetSQLValueString($_POST['status'], "int"),
               GetSQLValueString($_POST['shipping_band'], "text"),
               GetSQLValueString($_POST['price'], "double"),
               GetSQLValueString($_POST['product_id'], "int"));

这是我的表单代码：

    <form method="post" name="form1" action="<?php echo $editFormAction; ?>" enctype="multipart/form-data">
  <table align="center">
    <tr valign="baseline">
        <td nowrap align="right">Product ID:</td>
      <td>
    <?php echo $row_RS_updateProducts['product_id']?>
  </td>
<tr>
<tr valign="baseline">
  <td nowrap align="right">Thumb:</td>
      <td><?php echo $row_RS_updateProducts['thumb']?><input type="file" name="thumb" value="<?php echo     $row_RS_updateProducts['thumb']?>" size="32"></td>
</tr>
<tr valign="baseline">
  <td nowrap align="right">Image 1:</td>
  <td><?php echo $row_RS_updateProducts['image_1']?><input type="file" name="image_1" value="<?php echo $row_RS_updateProducts['image_1']?>" size="32"></td>
</tr>
<tr valign="baseline">
  <td nowrap align="right">Image 2:</td>
  <td><?php echo $row_RS_updateProducts['image_2']?><input type="file" name="image_2" value="<?php echo $row_RS_updateProducts['image_2']?>" size="32"></td>
</tr>
<tr valign="baseline">
  <td nowrap align="right">Title:</td>
  <td><input type="text" name="title" value="<?php echo htmlentities($row_RS_updateProducts['title'], ENT_COMPAT, 'UTF-8'); ?>" size="32"></td>
</tr>
<tr valign="baseline">
  <td nowrap align="right">Description:</td>
  <td><input type="text" name="desc" value="<?php echo htmlentities($row_RS_updateProducts['desc'], ENT_COMPAT, 'UTF-8'); ?>" size="32"></td>
</tr>
<tr valign="baseline">
  <td nowrap align="right">Category:</td>
  <td><select name="category_id">
    <?php 
do {  
?>
    <option value="<?php echo $row_RS_Category['category_id']?>" <?php if (!(strcmp($row_RS_Category['category_id'], htmlentities($row_RS_updateProducts['category_id'], ENT_COMPAT, 'UTF-8')))) {echo "SELECTED";} ?>><?php echo $row_RS_Category['category_name']?></option>
    <?php
} while ($row_RS_Category = mysql_fetch_assoc($RS_Category));
?>
  </select></td>
<tr>
<tr valign="baseline">
  <td nowrap align="right">Status:</td>
  <td><select name="status">
    <option value="1" <?php if (!(strcmp(1, htmlentities($row_RS_updateProducts['status'], ENT_COMPAT, 'UTF-8')))) {echo "SELECTED";} ?>>Available</option>
    <option value="2" <?php if (!(strcmp(2, htmlentities($row_RS_updateProducts['status'], ENT_COMPAT, 'UTF-8')))) {echo "SELECTED";} ?>>Pending</option>
    <option value="3" <?php if (!(strcmp(3, htmlentities($row_RS_updateProducts['status'], ENT_COMPAT, 'UTF-8')))) {echo "SELECTED";} ?>>Unavailable</option>
  </select></td>
</tr>
<tr valign="baseline">
  <td nowrap align="right">Shipping Band:</td>
  <td><select name="shipping_band">
    <option value="a" <?php if (!(strcmp("a", htmlentities($row_RS_updateProducts['shipping_band'], ENT_COMPAT, 'UTF-8')))) {echo "SELECTED";} ?>>A</option>
    <option value="b" <?php if (!(strcmp("b", htmlentities($row_RS_updateProducts['shipping_band'], ENT_COMPAT, 'UTF-8')))) {echo "SELECTED";} ?>>B</option>
    <option value="c" <?php if (!(strcmp("c", htmlentities($row_RS_updateProducts['shipping_band'], ENT_COMPAT, 'UTF-8')))) {echo "SELECTED";} ?>>C</option>
    <option value="d" <?php if (!(strcmp("d", htmlentities($row_RS_updateProducts['shipping_band'], ENT_COMPAT, 'UTF-8')))) {echo "SELECTED";} ?>>D</option>
  </select></td>
</tr>
<tr valign="baseline">
  <td nowrap align="right">Price:</td>
  <td><input type="text" name="price" value="<?php echo htmlentities($row_RS_updateProducts['price'], ENT_COMPAT, 'UTF-8'); ?>" size="32"></td>
</tr>
<tr valign="baseline">
  <td nowrap align="right">&nbsp;</td>
  <td><input type="submit" value="Update record"></td>
</tr>
  </table>
  <input type="hidden" name="MM_update" value="form1">
  <input type="hidden" name="product_id" value="<?php echo $row_RS_updateProducts['product_id']; ?>">
 </form>